A stellar eclipse worth watching

By Joe Rao

updated 7:44 p.m. ET Feb. 25, 2005

As the moon makes its monthly circuit around the sky it often passes in front of stars, blotting them out for as much as an hour or so.

Such an event is called an occultation (derived from the Latin word occultare, which means “to conceal”), and it can be a startling spectacle, especially if the star happens to be bright. The star appears to creep up to the moon’s limb, hangs on the edge for a minute or two, and then, without warning, abruptly winks out. Later it pops back into view just as suddenly on the moon’s other side.

The suddenness with which occultations take place was one of the first proofs that the moon has no atmosphere. If our natural satellite were cloaked with an atmosphere, a star approaching its limb would appear to fade away gradually, just like one seen setting beyond the western horizon from here on the Earth.

Early on the morning of Thursday, March 3, the Last Quarter moon will occult the 1st-magnitude red supergiant star Antares, one of the brightest stars in the sky. This should be a superb event for skywatchers in much of central and western North America.

Region of visibility
The action begins when Antares disappears behind the moon’s bright limb. This event will probably require a small telescope, because glare from the moon’s sunlit edge will tend to hide the star.

For those living in the northwestern United States as well as western and southern Canada, the moon may be barely above the horizon, so poor seeing -- atmospheric turbulence -- will compound the problem. Over the Pacific Northwest, Antares’ disappearance will not be visible because moon and star will have not yet risen.

For much of eastern North America, it’s a different problem: morning twilight will already be in progress, while unfortunately for Cape Cod, Massachusetts, Maine and the Canadian Maritime Provinces the sun will have already come up when the moon covers Antares.

The star’s reappearance will be spectacular wherever the moon is up in a dark sky.

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